Is there any RAW preventing me choosing a mythic creature's form with a polymorph spell? I'm not asking about unique named creatures, just the generic ones like a Myrmecoleon.
Depends. To quote Mythic Adventures, "[Mythic] creatures fall into one of two categories: powerful versions of existing monsters and entirely new breeds of monsters." The second type, of which Myrmecoleon is an example, is indeed a legal form to polymorph into. The first type (example mythic griffon) falls under the following rule, and is thus not allowed: "Polymorph spells cannot be used to assume the form of (...) an advanced version of a creature." (CRB pg. 212)
Note: The rule "cannot be used to change into specific individuals" refers to looks, not what creature you can sue as a base for your polymorphing. That means RAW you can take the form of a unique creature entry, although you have to seperate base creature and class levels/mythic stuff/feats/etc. as normal, and it's possible the creature falls under the above quoted "advances version" rule.